The main use of my site is video browsing. There is a search bar in the nav to search for videos. My site also has a photo section.
I need to add search functionality on the photo section. I'm torn between:

Having 2 search bars. The one in the nav for videos and one towards the top when browsing the photo sections.

When browsing the photo section, the nav search bar only searches photos. (Don't think this is a good idea..)

A multiple select search.. So a dropdown next to the search bar that lets users toggle between video and photo search. When browsing photos the default selected is photos.

Two search bars isn't as bad as it might sound as long as they're clearly separate; see the Users page which has a clearly separate Users search bar vs the main site search bar...and also the clearly even more separate Address Bar of your browser!
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Ben Brocka♦Apr 26 '12 at 16:47

A related example is that YouTube has 2 search boxes when viewing a user's channel. The top one is in the header and is clearly site-wide search. The smaller one is clearly labeled "Search Channel."
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Steven NotoApr 26 '12 at 22:33

A downside to comboboxes is that they hide the non-default values behind a click, and people in a hurry may miss them entirely. Then again Amazon uses this approach, so as long as the default choice is tailored to be what users "usually" want, it may be fine.
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Steven NotoApr 26 '12 at 22:35

There has been testing done for this, and there should never be more than one search box for content. Scoped searches can be useful, as long as you provide clear feedback on the scope being searched in the box and on the results; drcyrus3d's example looks like it does this well. However, users do not consider an address book 'content', so you may want a separate search box for the photo search if it feels like a search for a user, not a search for content. This may require some testing to see how your user base perceives this search type.

In addition to Karen's mockup, the search box could look like the mockup below. This will allow the user to choose what they are searching for without any extra steps to get the search results. If they don't choose whether they want Photos or Videos, Karen's solution would fit. Ultimately, functioning like Google Instant Search.

The first Search box is the default search box. The second one is an example of what a search would look like. Options to search for photos/videos are made available in the drop down, which is activated when the cursor is detected in the search field.